In the base of a small glass or ceramic baking dish, mix the lime zest and juice, green onions, garlic, cilantro, olive oil, coriander, cumin, chile powder, salt, and pepper. Use the tip of your chef’s knife to score the flank steak lightly on both sides and place in the baking dish on top of the marinade. Make sure the meat is well coated by turning it in the marinade.
Marinate in the refrigerator for 12-24 hours or set out the meat and marinade at room temperature for 20-30 minutes. Turn meat half way through marinating.
Preheat grill to medium-high.
Remove the steak from the marinade and discard the extra marinade. Grill steak, 3 minutes per side for medium rare or 5-7 minutes per side for medium to medium-well.
Transfer the steak to a cutting board. Tent with foil and let rest for 5-10 minutes before slicing thin against the grain. Finish with a sprinkling of course ground salt, such as Maldon, and serve with extra lime on the side.